Method of and apparatus for controlling a refrigeration machine



an. MILLER REFRIGERATION MACHINE Filed Jan. 17, 1968 April 28, 1970METHOD OF AND APPARATUS FOR CONTROLLING A INVENTOR. CHARLIE D. MILLER.BY Z ATTORNEY.

United States Patent Office 3,508,416 Patented Apr. 28, 1970 3,508,416METHOD OF AND APPARATUS FOR CON- TROLLING A REFRIGERATION MACHINECharlie D. Miller, 141 Mosley Drive, Syracuse, N.Y. 13206 Filed Jan. 17,1968, Ser. No. 698,552 Int. Cl. F25b 43/02 US. Cl. 62--84 6 ClaimsABSTRACT OF THE DISCLOSURE A control for regulating the operation of acapacity control device employed with centrifugal refrigerationmachines. The control includes means responsive to the pressuredeveloped by the lubricant pump for regulating the operation of thecapacity control system of the machine to either reduce or eliminatefoaming of refrigerant in the lubricant sump.

This invention relates generally to refrigeration machinery. Moreparticularly, the invention relates to refrigeration machinery employinga centrifugal refrigerant compressor equipped with guide vanes forvarying the capacity of the machine. Still more particularly, theinvention pertains to a control system for use with equipment of thekind described in which the control system incorporates means forgoverning the operation of the machine to avoid excessive foaming ofrefrigerant in the lubrication sump.

BACKGROUND OF THE INVENTION In the operation of relatively largerefrigeration machines having a capacity in range of 100-350 tons andemploying electric motor driven centrifugal refrigerant gas compressors,problems are encountered in assuring adequate lubrication of the movingparts and the co rresponding stationary parts that support the movingparts such as bearings. A major cause of these problems is the rapidvaporization of refrigerant, that has migrated from the refrigerant flowcircuit into the lubricant sump and mixed in the liquid state with thelubricant, as the pressure varies in the sump due to changes in machineoperating conditions. In some instances the vaporization of the admixedrefrigerant results in insufficient lubricant passing to the lubricationcircuit by the lubricant pump.

The lubricant sump is in communication with the low pressure side of themachine in order that the vaporized refrigerant may return to therefrigerant flow circuit. Accordingly, the vaporization of therefrigerant in the sump responds to the presusre fluctuations in the lowpressure side of the system and the temperature conditions in the sump.The usual practice, as far as the temperature is concerned, is toprovide a heater in the sump to maintain the temperature at a relativelyhigh level compared to the ambient during the period when the machine isinoperative. When the machine is inactive the pressure equalizes so thatan equilibrium condition dependent upon the ambient temperature isobtained. At this equilibrium condition there is refrigerant in theliquid state (admixed with the lubricant) and refrigerant in the vaporstate. Elevating the temperature of the body of liquid discouragesabsorption of the refrigerant by the lubricant at the pressure levelencountered at shut down.

Control over the pressure in the low side of the machine is usuallyaccomplished by restricting flow of refrigerant to the compressor byadjustable guide vanes or a suction damper. Thus the capacity of themachine is balanced with respect to the demand on the machine forrefrigeration, and at the same time the pressure is prevented fromfalling to a value such that freezing conditions in the evaporator couldobtain. This latter feature is important when the machine is used tochill water for flow in a circuit forming part of an air conditioninginstallation.

When the machine is started the chilled water is usually at a relativelyhigh temperature so that the control system associated with the machineactuates the guide vanes to an open position in order to increase thepumping capacity of the machine and reduce the pressure in theevaporator. The reduction of pressure in the evaporator or low side ofthe machine is transmitted to the sump causing violent frothing orboiling of the mixtures due to the vaporization of the liquidrefrigerant mixed with the lubricant. The lubricant pump then passes amixture of lubricant and vaporized refrigerant to the lubricationcircuit. Often the quantity of lubricant entering the circuit from thedischarge of the pump is insuificient to provide lubrication. Thiscondition is characterized by a drop in the discharge pressure of thelubricant pump. The reduction in pressure is not so severe as to tripthe minimum oil pressure safety switch and so operation of the machinecontinues although insufficient lubricant is being supplied to thelubrication circuit.

While one solution to this problem may be to elevate the set point ofthe minimum oil pressure safety switch this is not satisfactory becauseof the necessity to delay restarting after the machine is taken off theline. It will be appreciated that the current draw to the motor at startup is particularly heavy causing the windings to be heated to asubstantial degree. Accordingly the motor starting circuit is usuallyprovided with a time delay feature which prevents re-energization of themotor windings until the passage of a period of time sufficient for thewindings to cool down.

The chief object of this invention is the provision of a control forregulating the operation of a refrigeration machine wherein adequatelubrication of the parts is assured during periods of operation abovethe minimum oil pressure safety level.

Another object of the invention is the provision of a control forregulating the operation of a refrigeration machine that maintains theebullition of refrigerant admixed with lubricant in the sump of thecompressor within predetermined limits so as to assure adequatelubrication of the machine particularly at the time operation of themachine is initiated.

As pointed out above, centrifugal refrigeration machines are providedwith a control system for regulating the position of the guide vanesthroughout the operating range of a machine. This invention involvesmeans for overriding the action of the control system described underthose circumstances where it is indicated that delivery ofrefrigerant-free lubricant or a sufiicient lubricant to the lubricationcircuit may not obtain. To this end, the control system serving as thesubject of this invention acts to raise the pressure in the low pressureside of the machine so that the pressure in the lubrication sump is alsoelevated. With the pressure in the sump elevated, flashing of liquidrefrigerant in the mixture is prevented. This is accomplished by closingthe guide vanes to restrict the flow of gaseous refrigerant to thecompressor and is effected in a preferred embodiment by a circuitincorporating a switch responsive to a difference in the pressure oflubricant discharged by the lubricant pump and the suction pressure inthe machine.

Previous suggestions for accomplishing the described vane movement aredisclosed in United States Letters Patent 3,200,603, assigned to theassignee of this invention. The ararngement proposed here is animprovement over those disclosed in the patent for it eliminates thefloat members required in the patented control and is responsive to adifferent machine operating characteristic (lubricant pump pressure). Byutilizing the lubricant pump pressure a more reliable and less expensivearrangement is obtained.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrated in the drawingis a schematic representation of a centrifugal refrigeration machineequipped with a control forming the subject of the invention in whichcertain of the operating parts of the centrifugal compressor are shownin section together with a portion of the control system employed inconjunction with the machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT A centrifugal refrigerationmachine of the kind to which this invention applies is shown in part inthe drawing. The machine includes an impeller 12 mounted for rotationwithin a casing including the members 14, 16 and 18. The impeller 12 isdriven by an electric motor, not shown, secured to shaft 20, through agear reduction unit 22. Located within the casing is a sump 24accommodating a body of lubricant. Lubricant is forwarded from the sumpthrough pump 26 to a network of passages communicating with the portionsof the machine such as the seals and bearings requiring lubricationWhile the pump is illustrated as being remote from the interior of thecasing, it will be appreciated that lubrication pumps of the kind underconsideration are often secured to the main power shaft of the machineso as to be actuated simultaneously with energization of the motorcontrolling rotation of the shaft 20.

The impeller receives gaseous refrigerant through the inlet thereof andforwards same under elevated pressure to a discharge line connected witha volute section 28 formed in th casing part 16. The gaseous refrigerantflowing in discharge line 30 is converted to liquid refrigerant in thecondenser 32 as it rejects its heat of condensation to a cooling mediumflowing in coil 34. Liquid refrigerant formed in condenser 32 is passedvia refrigerant expansion device 36 to the evaporator 38 where it isconverted to the gaseous phase as it absorbs heat from the source ofchilled water flowing through coil 40 disposed within the evaporator 38.The gaseous refrigerant passes from the evaporator 38 through a suctionline 42 to a suction inlet member 44 in which a series ofcircu-mferentially spaced guide vanes 46 are mounted. After flow throughthe guide vanes, the gaseous refrigerant passes to the inlet of thecompressor completing the refrigerant flow cycle.

The lubrication sump 24 is in communication with suction line 42 throughconduit 48. Thus, it will be appreciated that the pressure within thelubricant sump is substantially the same as that present in suction line42 upstream of the guide vanes 46. Water flowing through coil 40,disposed within evaporator 38, forms a part of a closed water flowcircuit of an air conditioning installation. The chilled water absorbsheat after leaving evaporator 38 so that its temperature, upon return tothe evaporator, is considerably above its temperature upon departuretherefrom. The extent to which the temperature of the 64,66 respectivelyconnected across a suitable source of power. Connected in series withsecondary coil 60 in one 'leg of thebridge circuit is a resistor 70forth'e purpose of chilled water rises as it flows in the circuit is anindication of the refrigeration demand being made on the machine. Inorder that the capacity of the refrigeration machine be adjusted toaccommodate a variation in refrigeration demand, mechanism is providedfor positioning guide vanes 46 to either restrict or increase the volumeof gaseous r refrigerant flow passing through the compressor. Rotationof .the guide vanes 46 to either increase or decrease the amount of gasrefrigerant presented to the compressor is accomplished through alinkage 50 shown schematically in connection with the figure presentedherewith. The linkbalancing the bridge. The leg of the bridge circuitcontaining secondary coil 62 includes a variable resistor 72 known as athermistor. The thermistor is arranged to sense the temperature of thewater flowing in chilled watencoil 40 as it leaves the evaporator 38.Its resistance is a function of the temperature of the water and thuschanges with a variation in the temperature of water. Connected inparallel with thermistor 72 is a switch 74 for a purpose to be laterdescribed. Potentiometer 76 is providedin the bridge circuit for thepurpose of adjusting the temperature set point of the chilled water.Potentiometer 78 together with resistor 80 serve as a calibrationadjustment of the bridge circuit to compensate for the differenttolerances of the bridge componentsflResistor 80 is used as a final trimbalance of the bridge. Amplifier 82 is connected across the output ofthe bridge circuit and is employed to provide an energy source foreither relay 84,0r 86 depending upon the characteristic of the signalrepresenting the output of-the bridge circuit. The amplifier 82 iseffective to cause selective energization of relays 84 and 86.'

Considering the operationof the control circuit, it will be appreciatedthat under normal operating conditions the bridge'circuit is effectiveto supply a signal to the amplifier 82. The amplifier in accordance withthe'electrical char acteristic signal will energize either relay 84 or86 to complete a circuit through the coils of the shaded pole motor M toprovide movement of mechanism 50 to either open or close the vanesdepending upon the direction of variation of the temperature of thechilled water, it being understood the vanes will close upon a reductionin refrigeration demand and open upon an increase 'in refrigerationdemand.

At the time the machine, illustrated in the preferred embodiment, isstarted the control system is first operative to energize the chilledwater pump, not shown, controlling the flow of water through the chilledwater circuit including coil 40 and the lubricant pump in advance ofthemotor driving the compressor. The flow of relatively high temperaturewater through the coil 40 represents a large load on the machine so thatcontrol element 72 creates on output signal from the bridge circuiturging the motor M to rotate the linkage 50 to open the guide vanes 46and increasethe capacity of the machine to balance the load uponenergization of the impeller.

Pump 26 eventually develops sufiicient pressure to start the motordriving impeller 12. It also develops sufiicient pressure to causeswitch 74 to open an permit the bridge circuit to becontrolled fromthermistor 72. As pointed out above, the vanes 46 begin to move to anopen position. Within a relatively short period of time, the pressure inthe low side of the refrigeration machine and the compressor sump isreduced. Some vaporization of the refrigerant mixed with the lubricantin the sump occurs as the mixture achieves equilibrium under thenew-temperature-pressure relationship in the sump. The vaporizationcontinues until it affects the discharge pressure in the lubricant pump.Ultimately this pressure is reduced below theset point for actuation ofswitch 74, causing the switch to close and shunt out sensor 72 in thebridge circuit. At this point,'-the amplifier 82 receives a signalcausing energization of the relay' 84 to complete a circuit throughcoils 52 and 54 to actuate motor M to cause the guide vanes to close.The pressure in the low side of the machine and in the compressor sumpwill temporarily rise and terminate the vaporization of the admixedliquid refrigerant present in the lubricant. I

h In response to this condition, lubricant discharge pressure from pump26 will increase and subsequently cause movement of. the switch 74 to'an open position. The bridge circuit is againunder control of .sensor72 and. the vanes move to an open position in themanner described above.The reduction in sump pressure again oceurswith the consequent switchmovement as described above causing the vanes to close. The abovedescribed cycle is repeated as the refrigerant is alternately vaporizedand equilibrium conditions are re-established until vaporization ofrefrigerant no longer adversely affects the lubricant pump dischargepressure. Thus, an adequate flow of lubricant to the lubrication flowcircuit is provided.

While I have described a preferred embodiment of the invention, it is tobe understood the invention is not limited thereto since it may beotherwise embodied within the scope of the following claims.

I claim:

1. The method of operating a refrigeration machine including a lubricantflow circuit, a sump for accommodating lubricant, a pump for supplyinglubricant to the circuit and a capacity control system for regulatingrefrigerant flow to a refrigerant compressor which consists in the stepsof:

varying the amount of refrigerant passing to the compressor in responseto the load imposed on the machine; and

regulating the variation in refrigerant flow to the compressor inresponse to the pressure developed by said lubricant pump wherebyrefrigerant flow to the compressor is restricted to elevate the pressurein the compressor sump to inhibit vaporization of refrigerant present inthe lubricant.

2. The method set forth in claim 1 wherein the step of regulating thevariation of the refrigerant flow is responsive to a predeterminedpressure difference between the pressure of lubricant flowing from thelubricant pump and the suction pressure in the refrigeration machine.

3. Refrigeration apparatus comprising a compressor, a condenser,refrigerant expansion means, an evaporator and a suction line forsupplying refrigerant from the evaporator to the compressor, connectedto form a closed circuit for the flow of refrigerant, a lubricationsystem including pump means for supplying lubricant to the parts of thecompressor requiring lubrication, capacity control means includingadjustable refrigerant flow restricting means regulating flow ofrefrigerant to the compressor to vary the capacity of the apparatus,means for adjusting said flow restricting means in response to a changein demand for refrigeration and means operable during operation of saidlubricant pump responsive to predeteranined variation in the pressure oflubricant in the discharge side of said lubricant pump for regulatingsaid adjustable refrigerant flow restricting means independently of thechange in refrigeration demand on the machine.

4. Apparatus as set forth in claim 3 wherein said last mentioned meansis responsive to a predetermined difference in pressure between thelubricant discharged from said lubricant pump and the suction pressurein said apparatus. 7

5. Apparatus as set forth in claim 3 wherein said adjusting meansincludes a bridge circuit, the output of which responds to a variationin refrigeration demand.

6. Apparatus as set forth in claim 5 wherein switch means are providedin parallel in said bridge circuit, said switch means being effectiveupon energization to control the output of said bridge circuitindependent of said variation in refrigeration demand.

References Cited UNITED STATES PATENTS 2,199,426 5/1940 Wolfert 62l93 X2,983,111 5/1961 Miner et al. 62-217 X 3,081,604 3/1963 Namisniak et al.62192 X 3,200,603 8/1965 Wake et al. 62-84 WILLIAM E. WAYNER, PrimaryExaminer U.S. Cl. X.R.

